Reading Panel Specification Sheets

Every Solar Panel not made in Jim's basement is UL listed, meaning that it contains lots of data pertaining to how it performs in a laboratory.  This article is going to explain what all that stuff means.

Here is the spec sheet from one of my favorite solar panels, the Sunpower 230-watt panel (*Spr230).

Underneath "Electrical Data" you're going to find that it says "Measured at Standard Test Conditions (STC) irradiance of 1000/m2, air mass 1.5g, and cell temperature of 25°C."  It's basically the conditions under which all solar panels are tested in order to get unbiased information so that their performance can be compared to those of other manufacturers.

So let's get into what all of these numbers and abbreviations stand for, and how to understand them.

Pmax is the maximum wattage of the panel in the conditions described (STC).
+/- 5% means that it could be 218w, or it might be 241w, and that's called module mismatch since not all solar panels are created with exactly the same identical properties.

Vmp is the voltage at maximum power, while the panels is under load (connected to a system).
Imp is the current (I) at maximum power, also while the panel is connected.

Voc is the open circuit voltage of a panel.  That's when you connect each lead to a voltage tester.  It's a theoretical voltage which could occur if something bad happened to the system.  VOC is used to determine the maximum number of panels that you can wire in series.

Isc is the short circuit current of the panel.  It's the reading you get, under STC conditions, where the negative lead is plugged into the positive lead of the panel and a current reading is taken.  This helps determine the size of the fuses that you will use for your system later on, and also helps you determine the size of your DC Disconnect.

Maximum System Voltage is a rating determined by UL, and it has more to do with code requirements than anything else.  Remember Voc?  It can't be greater than 600v in the US.

Series Fuse Rating is the size of fuse that is required for the string.

Peak Power per Unit Area has more to do with the amount of space on the panel being consumed by solar cells than anything else.

CEC PTC is the California Energy Commission's rating of the solar panel's ability to produce energy.  Most of the stuff on this website is with the STC ratings, not PTC which is a different test, although many rebate programs will account for the amount of money that can be granted to a project based on the PTC.

Temperature Coefficients tell how the power changes under different temperature situations.  in this case, the voltage increases by 132.5 millivolts (or .132v) per °C.  This statistic is also used wiht the Voc to determine the maximum number of panels in series.